1. Field of the Invention
The present invention generally relates to light deflectors, light deflection arrays, optical systems, image forming devices, and projection type image display apparatuses, whereby the direction of outgoing light compared to incident light is changed. For example, the present invention can be used for image forming devices such as electro-photographic type printers or copiers, or projection type image display apparatuses such as projectors or digital theater systems.
2. Description of the Related Art
A light deflection method, light deflector, manufacturing method of the light deflector, and a product where the light deflector is applied, previously invented by inventors of the present invention, are disclosed in Japanese Laid-Open Patent Application Publication No. 2004-78136.
More specifically, a light deflector whereby light deflection in a single axial direction or two axial directions is made is disclosed in the Japanese Laid-Open Patent Application Publication No. 2004-78136. In this light deflector, a plate-shaped member having a fixed part, namely a mirror, is closed in a space and tilt displacement with respect to a fulcrum member as a center is done by electrostatic attraction so that the light deflection is performed.
In addition, optical deflectors and optical deflection method (driving methods) in cases where a contact electric potential is applied to the plate-shaped member, namely the mirror, and the plate-shaped member is electrically floated (held aloft by electrostatic force), are also disclosed in the Japanese Laid-Open Patent Application Publication No. 2004-78136.
The main structure of such an optical deflector and a driving method are discussed below.
FIG. 1 shows a light deflector disclosed in the Japanese Laid-Open Patent Application Publication No. 2004-78136. In this light deflector the contact electric potential is applied to the plate-shaped member, namely the mirror. This light deflector has a structure where the light deflection is made in both directions of each of two axes for a total of four directions.
FIG. 1(a) is a plan view of the light deflector, FIG. 1(b) is a cross-sectional view taken at a line A-A′ of FIG. 1(a), FIG. 1(c) is a cross-sectional view taken at a line B-B′ of FIG. 1(a), and FIG. 1(d) is a cross-sectional view taken at a line C-C′ of FIG. 1(a). FIG. 1 illustrates a single light deflector of a light deflection array where plural light deflectors are two-dimensionally arranged.
Referring to FIG. 1, the light deflector has a substrate 101, plural control members 102, a fulcrum member 103, a plate-shaped member 104, plural electrodes 105a, 105b, 105c and 105d, and an insulation layer 106 (fulcrum member 103 and electrodes 105a to 105d are illustrated with transparency). The plural control members 102 have stoppers in upper parts thereof. The stoppers 102 are provided at corresponding ends of substrate 101. The fulcrum member 103 has a top end made of a conductive member and is provided in the upper side of substrate 101.
The plate-shaped member 104 does not have a fixed edge. In addition, the plate-shaped member 104 has a light reflection area and a conductive material layer having electro-conductivity in at least one part. The plate-shaped member 104 is deployed to move in the space between insulation layer 106 and fulcrum member 103, but has its movement stopped by the stoppers 102. Electric potential of the plate-shaped member 104 is given by contact with the fulcrum member 103.
The plural electrodes 105a, 105b, 105c and 105d are formed in corresponding areas on top of the substrate 101. In addition, the plural electrodes 105 face a conductive material layer of the plate-shaped member 104. Applying different electrical potentials to the electrodes 105 controls an inclination or slant angle of the plate-shaped member 104.
A contact portion 106, not disclosed in the Japanese Laid-Open Patent Application Publication No. 2004-78136, is provided for reducing the contact area when the plate-shaped member 104 comes in contact with the substrate 101 due to tilt displacement.
The light deflector is tilted and displaced in directions 1 through 4 as shown in FIG. 1(c) and FIG. 1(d) by a combination of electric potentials applied to the fulcrum member 103 and the electrodes 105a through 105d. Corresponding to this, for example, light incident from a direction perpendicular to the substrate 101 surface can be reflected in the directions 1 through 4. In addition, light incident from the directions 1 through 4 can be reflected in a direction perpendicular to the substrate 101.
FIG. 2 shows a relationship between combination of electric potential application and a tilt direction of the plate-shaped member 104. As shown in FIG. 2, the light deflection in four direction can be made by combining and applying two kinds of electric potentials, namely X(V) and O(V), to five electrodes.
The above-described light deflector has the following advantages:
(a) Control of an inclination corner of a mirror (plate-shaped member 104) is easy and stable with a dip angle which dip angle is determined by contact of a fulcrum member and a substrate by the plate-shaped member;
(b) Response speed, for inclining the plate-shaped member, of a film to turn fast at high speed by applying different electrical current potentials to the electrodes can be achieved;
(c) The plate-shaped member does not have a fixed edge, and thereby there is little long-term deterioration, and the plate-shaped member can be driven by a low voltage;
(d) There are few shocks by collision with a stopper as the stopper is small, and a lightweight plate-shaped member can be formed by a semiconductor process, so that there is little long-term deterioration;
(e) The on/off ratio of reflection light (the S/N ratio in picture machinery, the contrast ratio in picture machinery) can be improved by the constitution of a control member and a plate-shaped member and its light reflection area;
(f) Miniaturization and an integration are possible at low cost by employing a semiconductor process; and
(g) A light deflection of 2 dimensions of 1 axis and a light deflection of 3 dimensions of 2 axes are possible by disposing plural electrodes.
Thus, the above-discussed light deflector has a lot of advantages and is better than a conventional twisting type optical switch or a diffraction grating type optical switch.
In addition, an optical system using the light deflector as a projection type image display apparatus is disclosed in Japanese Laid-Open Patent Application Publication No. 2004-138881. In Japanese Laid-Open Patent Application Publication No. 2004-13888, an optical system using an optical deflection array making the light deflection in two axial/four directions is disclosed. More specifically, two optical systems are suggested.
In a first optical system, white light shaped as parallel light is incident from a right upper side on the light deflection array having two axial/four light deflection directions. The white light is deflected in a specific deflection direction by red color information of three primary colors. The light being deflected reaches an image display part via a color filter in a light path, a first field lens, a projection lens and a second field lens so that color information wherein a single light deflector is regarded as a single pixel is formed on the image display part. Other colors are also deflected in specific directions so as to be overlapped on the image display part in a time order and recognized as a color image. Since three color displaying, wherein a single light deflection array (display device) is used without a color wheel, can be made by using such an optical system, it is possible to provide a projection type image display apparatus having a simple structure at low cost.
In a second optical system, three light sources individually emitting light fluxes corresponding to three primary colors of image information are used. The light fluxes from three light sources are incident on a single light deflection array from different directions and are deflected by the light deflectors forming the light deflection arrays in a time order. Reflection directions of the light fluxes of the colors are perpendicular to a surface of the light deflection array. Each light flux is lead to a single projection lens in time order so as to be projected and displayed on the image display part. Since three color displaying, wherein a single light deflection array (display device) is used without a color wheel, can be made in the second optical system, it is possible to provide a projection type image display apparatus having a simple structure at a low cost.
In addition, Japanese Laid-Open Patent Application Publication No. 2004-286970 discloses a structure where arrangement of plural electrodes formed on the substrate is improved so that stability of a light deflection operation is improved. Japanese Laid-Open Patent Application Publication No. 2004-317744 discloses a technique wherein an electret member is formed by a plate-shaped member so that the number of electrodes is reduced, manufacturing cost for unifying with a driving system is reduced, and the size of an apparatus is made small. In addition, Japanese Laid-Open Patent Application Publication No. 2005-17799 discloses a technique wherein plural electrodes formed on the substrate of the light deflector are driven in a bipolar manner so that storage of electric charge in an insulation film is prevented and stability of light deflection operation is improved.
As discussed above, the light deflector shown in FIG. 1 accrues a lot of advantages by using the plate-shaped member 104 not having the fixed part as the mirror. However, the plate-shaped member 104 no having the fixed part comes in contact with members forming the light deflector in the light deflection operation such as the contact part 106, the control members 102 including a stopper part and the fulcrum member 103.
The plate-shaped member is tilted and displaced by an electrostatic attraction force generated between the plural electrodes formed on the substrate and the plate-shaped member so that the light deflection apparatus is deflected. The light deflection apparatus applies a force always pushing on the fulcrum member by the plate-shaped member due to the electrostatic attraction. The light deflection apparatus makes the contact part formed on the substrate incline and collide to apply a force and makes the control member including the stopper have irregular contact for applying a force.
FIG. 3 is a cross-sectional view of the light deflector taken along a line A-A′ of FIG. 1. In FIG. 3, forces acting on the substrate at the time of the light deflection operation are shown by white arrows. These forces change energy or heat to strain of the members and fix the plate-shaped member to the substrate. In order to tilt and displace the plate-shaped member beyond the fixing force, it is necessary to increase voltage applied to plural electrodes and this causes obstacles to a low voltage driving the light deflector.
Thus, there is a problem in not only the light deflector giving the electric potential to the plate-shaped member via the fulcrum member but also the light deflector displacing the electrically floating plate-shaped member. In addition, in the twisting type optical switch or a both sides fixed beam type optical switch, such fixing happens when a member formed on the mirror comes in contact with an optional member formed on the substrate at the time of the optical operations.
At the waiting time when the light deflector does not work, since the electric potential is not applied to plural electrodes 105 formed on the substrate, the electrostatic attraction force does not act between the electrodes 105 and the plate-shaped member 104 so that the plate-shaped member 104 may freely move and come in contact with the stopper 102. The plate-shaped member 104 may be fixed to the stopper 102, as corresponding to collision energy with the plate-shaped member 104, surface energy difference between the stopper 102 and the plate-shaped member 104, an area contacting the stopper 102, or the environment such as temperature or humidity where the light deflector is provided.
FIG. 4 is a cross-sectional view of the light deflector taken along a line A-A′ of FIG. 1. FIG. 4 schematically shows a state where the plate-shaped member 104 freely moves at the time of waiting so as to come in full contact with and be fixed to the stopper 102. In the conventional art, by giving different electric potentials to plural electrodes provided so as to face the plate-shaped member 104 at the beginning of working, electric potential is electrostatically induced in the plate-shaped member 104 fixed to the stopper 102 so that the electrostatic attraction force exceeding the fixing force is generated and the plate-shaped member 104 is attracted to the side of the substrate 101 (reset operation).
In order to make the light deflector minute and the light deflection array have high integration as corresponding to a requirement for high precision or low cost of recent optical systems, it is necessary to reduce the area of the plate-shaped member 104 inducing the mirror area. Since the area of plural electrodes 105 facing the plate-shaped member 104 is reduced corresponding to this, it is necessary to increase the electric potential (reset voltage) given to plural electrodes 105 at the time of the reset operation. Increase of the reset voltage may make low voltage driving of the light deflector difficult and increase the consumption of electric power of products using the light deflector such as the optical system, image forming device, and projection type image display apparatus.
As discussed above, the problem to be solved by the present invention is based on the force acting due to the light deflection operation of the mirror, the force causing the plate-shaped member 104 to be fixed to members formed on the substrate. The present invention prevents such fixing so that low voltage driving can be achieved.
Generally, fixing may be generated by the electrostatic force, a water cross-linking force or a force between molecules. The electrostatic force may be caused by electrostatic charge. The water cross-linking force depends on absorption of moisture on a surface or under environmental conditions at the time of light deflection operation. The force between molecules depends on a distance with a contacted part. The fixing problem for the present invention is caused by the force between the molecules. Generally, the contact part or the plate-shaped member forming the light deflector is a thin film formed by a sputtering method or a CVD method and has a surface roughness of several tens through several hundreds nm. The contact at the film having such a surface roughness may not be influenced by the fixing due to the force between the molecules because the distance is too long.
In the conventional art, in a case where material of the contact part 106 is made of an aluminum group metal such as Al-1 wt % Si-0.5 wt % Cu or Al-2 wt % Cu generally used in a semiconductor process, and the material of the plate-shaped member 104 is made of an aluminum metal such as Al-1 wt % Ti or pure Al having a high reflection rate, its melting point is approximately 660° C. and lower, its Young's modulus is 70 through 100 GPa and lower, and its hardness by a non-indentation method is 2 through 3 GPa and lower. Because of this, the energy is converted to heats or strain of a member of the contact part by the collision force at the contact part 106 by the light deflection operation and the pushing force at the fulcrum member 103. As a result of this, the forming member is deformed, the contact distance is short, the contact area is increased, and the fixing force is increased. The light deflection at the driving voltage of several volts is difficult and therefore application of the driving voltage equal to or greater than 10 V is necessary for beginning the light deflection operation by overcoming fixing.